1) Field of the Invention
The present invention relates to a fuel control method and apparatus for a combined plant, and a program for allowing a computer to execute the fuel control method for the combined plant. More particularly, the present invention relates to a fuel control method for a combined plant that is not affected by the engagement or disengagement of a clutch, a control apparatus for use therein, and a program for allowing a computer to execute the fuel control method for the combined plant.
2) Description of the Related Art
A single-shaft combined plant is constructed by connecting a gas turbine, a power generator, and a steam turbine via a single shaft. In recent years, a single-shaft combined plant in which a clutch is disposed in a shaft between the power generator and the steam turbine has come on the market of the single-shaft combined plants. Such a single-shaft combined plant with a clutch is featured in that two rotors can be connected to or disconnected from each other via the clutch, and the gas turbine and the steam turbine can be started or stopped independently.
In such a single-shaft combined plant with a clutch, the clutch is disengaged until steam generated from an exhaust gas boiler that utilizes exhaust gas supplied from the gas turbine can be charged into the steam turbine, and, only the gas turbine and the power generator are first started in use. Then, once the steam turbine reaches a predetermined revolution number, the clutch is engaged. To the contrary, when the combined plant is stopped, the steam to be supplied to the steam turbine is first decreased, the clutch is disengaged, and thereafter, a fuel control valve is throttled, thereby stopping the gas turbine.
FIG. 8 is a diagram showing the configuration of a control system for the single-shaft combined plant with the clutch. First, the case such that a control system called a load limiting control system is adopted is explained. A gas turbine 81 and a steam turbine 82 are uni-axially connected to each other via a power generator 83 and a clutch 84. An output of the gas turbine 81 is controlled based on a control output 88 obtained by adding a value of a PID controller 87 to a difference E between a target load set value 85 from a host computer and an actual load 86.
The control output 88 is converted into a lift of a fuel control valve 89, and thereby a fuel flow rate is adjusted. Finally, the output of the gas turbine is controlled. The target load set value 85 from the host computer is appropriately reviewed according to the situation of power consumption. At this time, if the target load set value 85 is rapidly changed, the temperature around a burner and a blade of the gas turbine is changed, and as a result, the gas turbine is broken. In view of this, a change rate limiter 90 for suppressing a change rate is inserted immediately after the operation of the target load set value 85.
FIG. 9 is a diagram showing the configuration of another control system for the single-shaft combined plant with the clutch. This system is called a governor control system. In the case of output control of a gas turbine 91 in this system, a difference between a target load set value 92 from a host computer and an actual load 93 is first taken, and then, a revolution number command (SPSET) is produced accordingly, to be stored in a memory M. The output is controlled based on a control output 95 obtained by adding a gain K to a difference between the revolution number command value and an actual revolution number 94 of the gas turbine 91. Incidentally, a change rate limiter 96 is disposed immediately after the operation of the target load set value, which is the same as that of the load limiting control system.
When the single-shaft combined plant with the clutch such controlled as described above is adopted, torque required for the start is decreased since only the gas turbine and the power generator are first started in the state in which the clutch is disengaged. Consequently, it is possible to dispense with a starting device or reduce a capacity of the device. Furthermore, while only the gas turbine and the power generator are started, the steam turbine can be rotated at a low speed so as to ignore a windage loss. During this period of time, no cooling steam is needed. Consequently, it is possible to dispense with an auxiliary boiler or reduce a capacity of the boiler.
However, in the single-shaft combined plant with the clutch, there arise problems described below when the clutch is engaged and disengaged.
Since the torque of the steam turbine is applied to the power generator at a dash when the clutch is engaged, an output (i.e., an actual load) from the power generator is rapidly increased at a dash (the clutch cannot be engaged unless the torque of the steam turbine is applied at a dash). At this time, although the target load is constant in the combined plant control system, the actual load is rapidly increased, so that the fuel control valve is throttled at a dash. Such a fuel control abruptly changes the combustion state of the gas turbine, thereby leading to a danger of damage on a burner or a misfire.
To the contrary to the phenomenon, the torque of the steam turbine is eliminated at a dash when the clutch is disengaged, although it is necessary to disengage the clutch while the combined plant is stopped, and therefore, the output (i.e., the actual load) from the power generator rapidly drops in an instant at a dash. In other words, the clutch is successfully disengaged only when the valve is closed in such a manner that the torque of the steam turbine is reduced at a dash. At this time, since the target load is constant and the actual load rapidly drops, the fuel control valve is released at a dash. Such fuel control may also cause a damage given to the burner.